The comparative study of indirect and direct gap (Si and ZnO, respectively) semiconductor nanowires (NWs) is reported. The NWs were grown on silicon substrates by gold-assisted self-assembly technique. SEM study demonstrated high-density of nanowires. HRTEM studies show that both types of NWs (Si and ZnO) are the naturally-grown heterostructures consisting of crystalline core and envelope. The variation of photoluminescence properties of NWs with cross section and/or the material of the samples is analyzed.